An electronic device may include one or more circuit boards. A typical circuit board is a planar board that mechanically supports electronic components. The electronic components may comprise, for example, resistors, capacitors, switches, batteries, and other more complex integrated circuit components, i.e. microprocessors. The circuit board typically comprises a dielectric material, for example, a plastic material.
The circuit board may include conductive traces on the surface for connecting the electronic components to each other. As electronic circuitry has become more complex, multilayer circuit boards with at least two electrically conductive pattern layers have been developed. Typically, the different conductive trace layers of a multilayer circuit board may be connected through vertically extending vias, which comprise conductive materials, for example, metal.
A typical multilayer circuit board may comprise a plurality of core layers with bonding layers therebetween affixing the adjacent core layers together. Each core layer typically includes a dielectric layer with electrically conductive pattern layers on the opposing major surfaces of the dielectric layer. Typically, during manufacture of the multilayer circuit boards, the core and bonding layers are stacked together and then heated (laminated) to cause the bonding layer to affix the adjacent core layers together.
For example, one device application for the multilayer circuit board comprises an electrical inductor. The device is typically formed with spiral inductive elements on a major surface of the multilayer circuit board. Of course, an increase in the number of spiral inductive elements results in a commensurate rise in the generated inductance. Accordingly, there is a desire to decrease the spiral spacing between elements to generate more inductance while consuming less real estate of the multilayer circuit board.
In some applications, polymers may be used as the substrate for the spiral inductive elements. They may provide for several desirable characteristics, such as less loss. Nevertheless, the manufacturing techniques for these polymers may place a minimum on the spacing of the spiral inductive elements, for example, spiral spacing greater than or equal to 50 μm. Additionally, these polymer approaches may suffer from reduced operational bandwidth and may become self-resonant at low frequencies, i.e. making the inductor unusable.
An approach to this problem is to manufacture the spiral inductive elements on a semiconductor substrate, such as silicon or glass, where manufacturing precision is greater and allows for reduced spiral spacing. Notwithstanding the greater manufacturing resolution of building the electrical inductor on semiconductor material, these approaches may suffer from less operating bandwidth and greater loss due to the electrical characteristics of semiconductor material. Also, the semiconductor approaches may also experience self-resonance as the number of spiral inductive elements increases due to the loading effects of the semiconductor. Another drawback may include the need to insert a thick insulating layer between the semiconductor layer and the spiraled inductive elements to prevent a DC shorting of the spiral inductive elements, which adds to cost and board size.
For example, one approach is disclosed in U.S. Pat. No. 7,551,052 to Jow et al. The electrical inductor includes a high permeability magnetic substrate, conductive traces formed on the substrate to form circular inductive spirals, and a via passing through the substrate and coupling the conductive traces with additional traces on the backside of the substrate.
U.S. Pat. No. 7,345,563 to Pavier discloses a multi-layer circuit board comprising a laminate layer, a conductive layer thereon, and a magnetic layer also on the laminate layer. The multi-layer circuit board also includes a plurality of spiral inductive elements on the magnetic layer.